Field of the Invention
The present invention relates to a synchronous network having a multiplicity of nodes that can transmit data to one another in a predefined sequence for a predefined duration.
Such a network is, for example, a network that operates according to what is referred to as the time division multiple access (TDMA) method.
A known network contains four units which are referred to below as nodes, and a transmission link which connects the nodes and via which data which is to be transmitted between the nodes can be transmitted.
For the sake of completeness, it is to be noted that the number of nodes is not restricted to four; it may be any desired larger or smaller number.
It is also to be noted that the transmission link may be of any desired configuration. It may be an electrical line that is used to transmit electrical signals, an optical waveguide that is used to transmit optical signals, a transmission channel which permits data to be transmitted in a wirefree fashion, or some other transmission link.
The nodes contain definitions of the sequence in which the respective nodes are allowed to transmit, and a width of the time slot during which they are allowed to transmit data.
A known problem in systems of this type consists in the fact that the nodes have to be synchronized after starting up, after faults, after changes of operating mode etc., and that it is necessary to ensure that they remain synchronized. Otherwise, the nodes may not transmit data precisely, or not at all, in the time slots reserved for them, and this may lead to a situation in which data originating from different nodes on the transmission link is superposed or overlaps and as a result becomes unusable.
The nodes are generally synchronized in accordance with one of the two methods described below.
A first synchronization method is defined by a reference clock being fed by one of the nodes to the other nodes, and that, by referring to the reference clock, the other nodes synchronize themselves with the node which transmits the reference clock. The method has the disadvantage that if the node that transmits the reference clock fails, the entire system fails.
A second possibility is that the nodes synchronize themselves by referring to a start and/or an end time and/or the duration of the transmission of data from one node to another node. The method has the disadvantage that after the system is started up it takes an indeterminately long time until the nodes are synchronized. The method of synchronization can in fact not take place until after a node has been able to transmit data without another node simultaneously transmitting data, and this is sometimes an awkward condition before the synchronization, that is to say at a time at which it has not yet been defined at what time which node is allowed to transmit.